To enable kernel debugging on openwrt, you must make sure：

1. serial port driver support poll_get_char and poll_put_char routine.
2. disabled cpu watchdog.

otherwise, you will failing in kernel debugging
1. unable to configure kgdboc
2. cpu reset when kernel debugging

Do as following

1.make kernel_menuconfig
enable kernel debugging sysrq etc

2. modify serial port driver  /build_dir/..../driver/tty/serial/ar933x_uart.c

modify struct uart_ops ar933x_uart_ops:

static struct uart_ops ar933x_uart_ops = {
    .tx_empty    = ar933x_uart_tx_empty,
    .set_mctrl    = ar933x_uart_set_mctrl,
    .get_mctrl    = ar933x_uart_get_mctrl,
    .stop_tx    = ar933x_uart_stop_tx,
    .start_tx    = ar933x_uart_start_tx,
    .stop_rx    = ar933x_uart_stop_rx,
    .enable_ms    = ar933x_uart_enable_ms,
    .break_ctl    = ar933x_uart_break_ctl,
    .startup    = ar933x_uart_startup,
    .shutdown    = ar933x_uart_shutdown,
    .set_termios    = ar933x_uart_set_termios,
    .type        = ar933x_uart_type,
    .release_port    = ar933x_uart_release_port,
    .request_port    = ar933x_uart_request_port,
    .config_port    = ar933x_uart_config_port,
    .verify_port    = ar933x_uart_verify_port,
#ifdef CONFIG_CONSOLE_POLL                                         //add this
    .poll_get_char = ar933x_uart_get_poll_char,                  //add this
    .poll_put_char = ar933x_uart_put_poll_char,                 //add this
#endif                                                                             //add this
};

3. add following code before struct ar933x_uart_ops

#ifdef CONFIG_CONSOLE_POLL
int is_shutdown_watchdog = 0;

#define RST_WATCHDOG_TIMER_CONTROL      0x18060008      //refer to AR9331 datasheet 6.6.3

void shutdown_watchdog(void)
{
    if(!is_shutdown_watchdog)
    {
        unsigned long *p = (unsigned long *)ioremap_nocache(RST_WATCHDOG_TIMER_CONTROL,8);

        if(p)
        {
            writel(0,p);        //Action = 00 (NO ACTION)
            iounmap(p);
            is_shutdown_watchdog = 1;
        }
    }
}

static int ar933x_uart_get_poll_char(struct ar933x_uart_port *up)
{
    unsigned int rdata;

    shutdown_watchdog();

    do {
        rdata = ar933x_uart_read(up, AR933X_UART_DATA_REG);
        cpu_relax();
    } while ((rdata & AR933X_UART_DATA_RX_CSR) == 0);
   
    /* remove the character from the FIFO */
    ar933x_uart_write(up, AR933X_UART_DATA_REG,AR933X_UART_DATA_RX_CSR);

    return rdata & AR933X_UART_DATA_TX_RX_MASK;
}

static void ar933x_uart_put_poll_char(struct ar933x_uart_port *up, unsigned char c)
{
    unsigned int rdata;

    do {
        rdata = ar933x_uart_read(up, AR933X_UART_DATA_REG);
        cpu_relax();
    } while ((rdata & AR933X_UART_DATA_TX_CSR) == 0);

    ar933x_uart_putc(up, c);
}
#endif

now make openwrt
and try this

echo ttyATH0 > /sys/module/kgdboc/parameters/kgdboc
echo g > /proc/sysrq-trigger

type kgdboc protocol cmd:
$g#67

you will successfully communicate with kgdbstub

try you gdb client
gdb vmlinux
set remotebaud 115200
target remote /dev/ttyUSB0

have a fun!

(Last edited by mengxp on 25 Oct 2014, 05:58)

Good.